The growth of III-V on Si has been recognized as a highly desirable technology goal for a number of years. The earliest work focused on GaAs solar cells because they were very large area devices where the substrate cost, maximum substrate size, ruggedness and weight (for space applications) were major obstacles for conventional GaAs homoepitaxial approaches. Si substrates provided an attractive solution to all of the above problems. In addition, the alloy composition of AlGaAs can be adjusted to provide an optimum bandgap and absorption match to Si for high efficiency multiple bandgap solar cells. Early efforts attempted the growth of GaAs directly on both single and polycrystal Si, but with very little success. This was not a surprising result because of the expected heteroepitaxial problems created by a 4% lattice mismatch, large thermal expansion mismatch and polar/non-polar interface with antiphase disorder, cross doping and phase segregation. These initial efforts then evolved into a variety of approaches utilizing refractory metals, Ge or Si-Ge graded layers with subsequent growth of GaAs. These structures still faced the polar/non-polar interface problems. While reasonably efficient GaAs on Ge single crystal cells were realized, the results with various interfacial layers on Si were not too encouraging.
About 1981, the potential advantages of large-size wafers, optical interconnects, optoelectronic integrated circuits (OEIC) and monolithic integration of ultra-high speed GaAs with high density Si VLSI pushed a re-examination of the earlier difficulties of direct GaAs/Si heteroepitaxy. There were three key results which have greatly changed the outcome from the prior failures. First was the ability to achieve a clean (relatively O and C free) Si surface. Second was the separation of the nucleation and growth phases in the two-step growth process. Third was the role of tilting the substrate off the direct [100]orientation to form an array of even atomic layer steps in the Si surface to eliminate formation of anti-phase domains. The results of these breakthroughs have made GaAs and other III-V materials on Si increasingly promising from the device and IC perspective.
Recently, nearly all types of GaAs and hetero junction devices have been demonstrated in GaAs/Si. For some applications, the device performance is comparable to conventional GaAs approaches, while for others, especially for optical devices, it is still inferior.
Despite the current progress in producing devices having III-V on Si, a need still exists for such devices having fewer defects and a reduction of the strain caused by the difference of the coefficients of thermal expansion between III-V materials such as GaAs and substrate materials such as Si.